This invention relates to a method of and apparatus for recording and/or reproducing video signals on a record medium and, more particularly, to a method and apparatus for recording video signals with a high recording density and for reproducing such signals with minimal interference in the displayed video picture due to crosstalk picked up from adjacent tracks when a particular track is reproduced.
In a typical video recording system, such as a video tape recorder (VTR), a video signal is recorded on a magnetic medium, such as magnetic tape, in successive, parallel, skewed tracks, each track generally having a field interval recorded therein and being formed of successive areas which correspond to respective line intervals of the video signal. If the video signal is a composite color television signal, recording is carried out by separating the chrominance and luminance components, frequency modulating the luminance component to a relatively higher band of frequencies, frequency converting the chrominance component to a band of frequencies which is lower than that contained in the frequency-modulated luminance signal, combining the frequency-modulated luminance signal and frequency-converted chrominance signal and recording the combined signal in the same track. In order to avoid interference due to crosstalk during a signal reproduction operation that is, to avoid interference due to signals which are picked up by a scanning transducer from an adjacent track when a given track is scanned, it has been the practice heretofore of providing guard bands to separate successive parallel tracks on the record medium. Such guard bands essentially are "empty" of information so as to avoid crosstalk pickup from such adjacent guard bands when a particular track is scanned.
However, the use of guard bands to separate successive tracks is a relatively inefficient usage of the record medium. That is, if the guard bands themselves could be provided with useful information, the overall recording density would be improved. Such improvement can be attained to some degree by providing two transducers for recording the combined luminance and chrominance signals, the two transducers having different azimuth angles. Hence, information is recorded in one track at one azimuth angle and information is recorded in the next adjacent track with a different azimuth angle. When the information in such tracks is reproduced by the same, respective transducers, the information recorded in the scanned track is reproduced with minimal attenuation, but because of azimuth loss, the crosstalk which is picked up from the next adjacent track is substantially attenuated. Since azimuth loss is proportional to the frequency of the recorded signals, it may be appreciated that the crosstalk due to the frequency-modulated luminance signals included in the recorded color television signals is far more attenuated than the crosstalk due to the frequency-converted chrominance signals. Also, since crosstalk attenuation due to azimuth loss is less effective as the width of the parallel tracks is reduced, it is not sufficient to rely solely on the use of transducers having different azimuth angles in order to reduce crosstalk noise when video signals are recorded in very narrow or overlapped tracks. If the crosstalk picked up from an adjacent track is not attenuated adequately, an interference or beat signal, having a frequency different from either the information signals which are recorded in the scanned track or the picked up signals which are recorded in an adjacent track, will appear as a beat or moire pattern in the video picture which ultimately is displayed.
Since reliance upon azimuth loss is not completely adequate for minimizing crosstalk interference caused by the frequency-converted chrominance signals which are picked up from an adjacent track, it has been proposed that such cross-talk can be reduced substantially by recording the frequency-converted chrominance signals in adjacent tracks with different carriers. For example, the phase of the frequency-converted chrominance carrier can be constant throughout successive line intervals in one track but will shift by 180.degree. from line-to-line in the next adjacent track. As another example, the phase of the frequency-converted chrominance carrier in alternate line intervals in one track will differ by 180.degree. (or .pi.) from the phase of the frequency-converted chrominance carrier in adjacent alternate line intervals in an adjacent track, while all of the remaining line intervals in adjacent tracks will have frequency-converted chrominance carriers which are in phase with each other. Because of these phase characteristics in both examples, the crosstalk interference due to the frequency-converted chrominance signals which are picked up from an adjacent track will exhibit a frequency interleaved relationship with respect to the frequency-converted chrominance signals which are reproduced from the scanned track. Suitable filtering techniques can be used to eliminate those frequency components which correspond to the crosstalk interference.
While the use of different frequency-converted chrominance carriers is an effective technique for minimizing crosstalk interference attributed to the chrominance signals, there still will be crosstalk interference due to the frequency-modulated luminance signals, particularly if the record tracks exhibit minimal width. One proposed solution to this problem is disclosed and claimed in copending application Ser. No. 770,315, filed Feb. 18, 1977, wherein different carriers for the frequency-modulated luminance signal are recorded in adjacent tracks. This is carried out by using two different bias voltages superposed onto the luminance signal prior to frequency modulation thereof, which bias voltages effectively determine the frequency of a frequency-modulated carrier. As one example of this proposed solution, the frequencies of the carriers differ from each other by an odd multiple of one-half the horizontal synchronizing frequency. In a signal reproduction operation, the reproduced frequency-modulated luminance signal is demodulated, and the bias voltages which had been added to the original luminance signal are removed therefrom, as by subtracting locally-generated bias voltages from the recovered luminance signal. When the reproduced signals are displayed, as on a cathode ray tube, crosstalk interference will be present in successive lines, but such interference will be phase-inverted from line-to-line. Hence, this crosstalk interference will cancel visually and will not be perceived by a viewer.
Another proposed solution is described in application Ser. No. 815,012, filed on even date herewith, wherein the phase of the frequency modulated luminance signal is selectively shifted by an odd multiple of .pi. during selected line and field intervals. For example, the phase of the frequency modulated luminance signal may be shifted in successive line intervals which are recorded in alternate tracks, while such phase remains constant in the remaining tracks. As another example, the phase of the frequency modulated luminance signal varies by an odd multiple of .pi. between alternate line intervals in one track and adjacent alternate line intervals in the next adjacent track.